This invention relates to means and methods for securing a pipe member and the like against axial rotation, and more particularly relates to improved back-up means and methods for connecting and disconnecting drill pipe of the sort commonly employed to drill oil and gas wells.
It is well known that oil and gas is found in subsurface earth formations, and that boreholes are drilled into these formations to recover these substances. What is not so well known, however, are the problems and difficulties which attend the drilling of such boreholes, and consequently the peculiarities and requirements which must be met.
In the first place, such wells are often thousands of feet deep in the earth, and therefore the mere drilling of such wells is a technological challenge, not only with respect to cutting away rock and soil of various character at different depths, but also the need to remove the drill cuttings being produced at the bottom of the borehole. Accordingly, it is now conventional to drill such wells using a bit or cutting tool which is suspended at the end of a tubular string of lengths of pipe. More particularly, the drill bit is rotated at the bottom of the borehold by rotation of the string of drill pipe at its upper end, while suitable drilling fluids or "muds" are pumped down the interior of the drill string and out through apertures in the drill bit.
The drilling mud or slurry serves various essential purposes, in addition to lubrication of the cutting surfaces of the drill bit. The mud which flows down the interior of the drill string, returns to the surface by way of the annular space between the wall of the borehole and the outside surface of the drill string, to carry away the drill cuttings which would otherwise accumulate in the borehole. Thus, the drilling mud also serves as a lubricant between the wall of the borehole and the string of revolving drill pipe. Furthermore, the weight of the column of drilling mud in the borehole provides a back-pressure in the event that the drill bit unexpectedly encounters a formation containing fluids under an abnormally high pressure.
The string of drill pipe is necessarily assembled at the surface on a piece-by-piece basis, wherein each length or "joint" of drill pipe is selectively connected to the upper end of the last joint previously added, as the string is progressively lowered into the borehole. Similarly, each time it becomes necessary to repair or replace the drill bit, (which occurs many times during the drilling of deeper boreholes), the string of pipe is progressively lifted out of the borehole as the topmost joint of pipe is removed.
Although there are many difficulties attending the drilling of a borehole in the earth, there are particular difficulties which relate to the making and breaking of the threaded couplings between the joints of drill pipe. Since drill pipe tends to lose wall thickness as it is revolved in the borehole, it is undesirable to apply any significant gripping force to a joint of drill pipe lest this effect a crushing which, in turn, tends to produce a longitudinal weakening in the pipe. On the other hand, it is essential that sufficient torque be applied to the pipe to achieve a fluid-tight connection between each two joints of pipe, to avoid a pressurized discharge of drilling fluid which erodes the wall of the borehole, and this in turn necessitates the application of a heavy gripping force. In an attempt to compromise these two conflicting needs, most drill pipe is now formed with a box-like portion at one end, the "box" being provided with extra-heavy wall thickness and with inside threads, and with an extra-heavy wall thickness immediately adjacent the exterior threads at the other end. Provision of a wall thickness of this magnitude will, even for drill pipe having a severely eroded exterior, permit the pipe to resist compressional forces which might otherwise crush the pipe but which are necessary to apply adequate torque to the pipe in making and breaking the threaded connections between two joints of drill pipe. Nevertheless, this has further necessitated that the pipe be seized only at the two end portions having an extra-heavy wall thickness, and under no circumstances at any other place along each joint of drill pipe.
Originally, the drill string was assembled and disassembled using two sets of manually operated pipe tongs, wherein one set was applied to the "box" portion of the pipe at the upper end of the drill string in the borehole, and the other set was applied to the lower end (immediately above the threads) of the pipe being added or removed from the drill string.
The upper set of tongs was conventionally interconnected by a cable and torque gauge to a powered winch, and the lower set of tongs was connected oppositely thereof through a cable to a point of anchorage on the drilling rig. When the winch was activated, the connecting cable would turn the so-called "pipe tongs" to rotate the upper joint until the torque gauge registered the torque considered to effect a fluid-tight seal, (during make-up of the drill string), or to break the threaded connection in the case of disassembly of the drill string. The snubbing cable attached to the lower tongs would, of course, secure the lower tongs and thus the box portion of the lower joint of drill pipe from rotation during this process.
It will readily be apparent that such a process was time consuming and therefore expensive. More particularly, however, it was often extremely dangerous because of the possibility of cable breakage, and this possibility was increased when the so-called "manual" tongs used to rotate the upper pipe joint were replaced by hydraulically-actuated rotating tongs of the type depicted in U.S. Pat. No. 4,084,453, in order to achieve even higher torques prescribed for threaded connections in deeper boreholes.
Another disadvantage to such techniques, even after hydraulically-actuated rotary tongs came into widespread use, arose out of the inaccuracy of the measurement being provided by the torque gauge now interconnected with the snubbing cable. More particularly, it was long recognized that the torque gauge would accurately indicate the torque on the pipe only when the two cables were positioned to define force vectors which, in turn, were positioned at exactly 90 degrees of each other, and that this condition would only exist momentarily as the upper pipe was revolved. Thus, the problem of achieving a fluid-tight seal between adjacent lengths of drill pipe continued to exist even after the adoption of powered rotary tongs.
In some instances it has been found useful to couple the so-called "manual" back-up tong to the powered rotary tong assembly, whereby the two sets of tongs might be handled and operated as a unit, and whereby the drill string could be more quickly and conveniently connected and disconnected. In such an arrangement, of course, the two tongs tend to be snubbed together whereby at least one of the two cables were eliminated. Even this has become impractical with the need to develop increasingly higher torques by the rotary tong assembly, however, which were beyond the gripping capability of conventionally designed "manually-operated" tongs. Accordingly, there has long existed a need for powered back-up tongs which can develop a gripping force on the box portion of the drill pipe which is capable of immobilizing the drill string against the torques now required to obtain a fluid-tight pipe connection.
There have, of course, been many attempts to develop powered back-up tong assemblies having this capability, although all such attempts have been less than completely successful. One significant problem, which has prevented the use of designs similar to those incorporated in the rotary powered tong assemblies, has been the need to locate the back-up tongs at the box of the lower drill pipe and therefore immediately below and in close proximity to the lower surface of the rotary powered tongs. Another problem arises from the fact that, although a combination of the rotary and back-up tongs may eliminate the snubbing cables, the combined tong assembly is itself a hazard in the event of unscheduled vertical movement of any portion of the drill string.
These and other disadvantages of the prior art are overcome with the present invention, and improved powered back-up tongs are herewith provided for interconnection and use with powered rotary tongs.